Significance and Objectives The objective of this study was to determine whether treatment with a GnRH antagonist would suppress gonadotropins and influence ovarian function when given during the mid-luteal phase (MLP). During the MLP, gonadotropin withdrawal induced by a GnRH antagonist should cause luteolysis thereby shortening the overall length of the luteal phase and menstrual cycle. In addition FSH dynamics during the luteal phase may be important for the normalcy of the next menstrual cycle. Therefore the effects of the GnRH antagonist on the subsequent menstrual cycles, specifically its effect on urinary FSH, timing of ovulation, length of follicular and luteal phases were also studied. Results Four non-pregnant monkeys received one treatment of 30 5g/kg of Azaline B by subcutaneous injection 8-10 days post ovulation. Daily urine samples were collected during the entire treatment cycle and continued through the two subsequent cycles in order to determine whether the effect of treatment carried over into the following cycle and its duration of effect. Daily urine samples were analyzed for E1C, PdG and FSH levels. Three of the four animals treated exhibited some effects of the treatment (i.e. shortened luteal phases and FSH suppression), however only one of these animals experienced a sustained suppression of FSH that led to a delay of ovulation in the subsequent cycle. Future Directions Azaline B, a potent GnRH antagonist, suppresses gonadotropins, specifically FSH, when administered during the MLP. However, a higher dose or repeated injections of Azaline B may be necessary to disrupt the subsequent cycle in the majority of animals. KEY WORDS cynomolgus macaque, Azaline B, GnRH antagonist FUNDING NIH Grant RR00169